The present invention relates to apparatus and methods for performing abrasive work on a work piece using an abrasive visco-elastic medium and, in particular, to apparatus and methods that impart a relative cyclic motion between the work piece and the medium to provide a separation between the medium and work piece during each cycle which separation is occupied by a fluid having a viscosity less than that of the visco-elastic medium and where the deformation of the medium is between 50 and 99%.
1. Technical Field
The present invention relates to the technical field of machining and particularly to the field of non-traditional machining processes and equipment employing the techniques of and compositions for abrasive flow machining, grinding, deburring, radiussing, leveling and polishing of work pieces. Such processes are typically employed in the working of castings, forged parts, machined parts, and the like. Most often metal parts and the like. The present invention particularly relates to such operations where the flow is attained by relative motion, preferably orbital motion, between the work piece and the abrasive medium.
2. Prior Art
Abrasive flow machining has gained wide acceptance for a number of applications as the machining and finishing technique of choice. Such techniques are particularly adapted, for example, to working interior passages in work pieces, for light grinding, deburring, radiussing leveling and polishing of complex surfaces, and particularly three-dimensioned surfaces where surface detail requires working, and in repetitive working of multiple work pieces of complex form and shape.
In its simplest form, abrasive flow machining requires passing a visco-elastic medium containing an abrasive across the surfaces to be worked. The visco-elastic medium functions as a carrier for the abrasive, and transmits working force to the abrasive as the abrasive is carried across the surface. The medium flows to conform to the surface of the work piece.
In many contexts, advantage is taken of the visco-elastic character of the medium to pump the abrasive filled medium through passages, across surfaces, and between a work piece surface and a suitable member to confine the flow and constrain the medium in engagement with the surface of the work piece.
In many contexts, particular advantages are attained when the visco-elastic abrasive medium is also rheopectic, i.e., increasing in apparent viscosity with applied stress. (The behavior of rheopectic materials is, in essence, the converse of thixotropic behavior.) With the appropriate application of stress, typically either shear or compressive stress, to the medium, it is possible to substantially attain plug flow of the medium across the surfaces of the work piece to be worked in the operation. Substantially higher working force is applied to the surface by such plug flow when compared to viscous flow of the medium.
A more detailed description of the basic prior art on orbital abrasive flow polishing and grinding can be found in U.S. Pat. Nos. 3,521,412, 3,634,973, McCarty and U.S. Pat. No. 3,819,343, Rhoades.
One particularly attractive implementation of abrasive flow machining has been the employment of an rheopectic visco-elastic abrasive medium in combination with an orbital drive mechanism, where the medium and a work piece are confined together in a pressurized chamber with a xe2x80x9cdisplacer memberxe2x80x9d generally conforming to the inverse of the shape of the work piece surfaces to be worked, and where the orbital motion of the work piece relative to the chamber displacer member and the contained medium causes the work piece to be effectively worked. Such operations take advantage of the ability of the medium to conform to the surfaces of the work piece, and even complex, highly detailed surface shapes can be worked with considerable success and effectiveness. See U.S. Pat. No. 5,125,191, Rhoades.
It is in the context of such abrasive machining to which the present invention particularly relates, although the considerations disclosed and described herein may have broader and more general applicability in specialized circumstances.
Orbital working with abrasive flow machining techniques have been employed with effectiveness. Such operations have been limited in some contexts, however, by several factors. First and foremost, the mode of operation in the prior work is based upon fluid or plastic flow of the medium across the surfaces of the work piece. The action is based on xe2x80x9cextrusionxe2x80x9d of the medium through a restricted gap between the work piece and the mating displacer or mandrel. Such flow does an excellent job of polishing surfaces at high speed, but for more demanding operations involving significant removal of stock from the work piece, the operation is typically slow. There is a need in the art for techniques for attaining adequate working at higher and more productive rates.
Second, the chamber in which the orbital working takes place must be closed or seated to confine the medium within the chamber, assuring adequate strain rates are attained and imparted to the medium to provide the required work, adding to the complexity of the equipment and the time and effort required to change work pieces. These requirements also contribute substantially to the cost of the process and equipment and may, in some cases, be a limiting factor determining the rate of production.
It is an object of the present invention to provide an improved abrasive machining process and apparatus in which a separation is maintained during the processing between the visco-elastic medium and portions of the work piece. It is another object of the present invention to provide an improved abrasive machining process and apparatus employing a visco-elastic abrasive medium which conforms to the shape and conformation of each work piece.
It is another object of the present invention to provide an improved abrasive machining process and apparatus employing a visco-elastic abrasive medium which performs abrasive work on each work piece predominantly by elastic deformation.
It is still another object of the present invention to provide an improved abrasive machining process and apparatus employing a visco-elastic abrasive medium with no requirement for a sealed working chamber.
Yet another object of the present invention is to simplify the requirements for mandrels and displacer elements employed in the system for abrasive machining, and in some cases to eliminate such requirements entirely.
It is yet another object of the present invention to provide an improved abrasive machining process and apparatus employing a visco-elastic abrasive medium for light grinding, deburring, radiussing, leveling and polishing of complex surfaces, and particularly three-dimensioned surfaces where surface detail requires working and repetitive working of multiple work pieces of complex form and shape.
These and still other objects, which are made apparent in the following disclosure and description of the invention, are attained in the present invention.
The present invention provides light grinding, deburring, radiussing, leveling and polishing of complex surfaces, and particularly three-dimensioned surfaces where surface detail requires working, and in repetitive working of multiple work pieces of complex form and shape. Generally, the apparatus of the present invention comprises a chamber which can be either closed or open for mounting a work piece. Preferably the new medium is fed to a gap, the chamber preferably includes a plurality of inlets for feeding a visco-elastic abrasive medium therein. A drive is provided for imparting relative motion between the work piece and the visco-elastic medium by contacting the visco-elastic medium so as to create a separation between the medium, and portions of the work piece. The separation is filled with a fluid of less viscosity than the medium, such as air. The separation is preferably maintained in the areas adjacent the inlets. In the preferred embodiment a cyclic motion is imparted by the drive which causes the medium to deform from 50 to 99% and preferably from about 80 to 95%. Deformation recovery rate is generally a function of the speed of cyclic rotation, preferably an orbital rotation.
The present invention is based on the practice of abrasive machining with no sealed working chamber by the employment of a visco-elastic abrasive medium, behaving predominantly as an elastic solid at the applied strain of the working motion, and applying orbital or other relative working motion to produce strain rates which bring the medium into a predominantly elastic deformation and often near, but not to exceed, the compressive stress limit at the strain rate employed. (The compressive stress limit generally corresponds to the xe2x80x9cupset valuexe2x80x9d for ductile metals, as determined by ASTM E9-89a, and is closely related to the buckling limit, barreling limit or the fracture limit for the material.)
The preferred visco-elastic abrasive medium is a rheopectic poly(boro-siloxane), filled with viscosity increasing stiffening agents and high loadings of the abrasive of choice, and relatively minor amounts of plasticizers. The preferred poly(boro-siloxane) will have a static viscosity of from about xcex7=5xc3x97103 Centipoise to about xcex7=5xc3x97105 Centipoise. The static viscosity of the formulated medium should be in the range of from about xcex7=2xc3x97104 Centipoise to about xcex7=8xc3x97106 Centipoise.
At the high static viscosities and even higher apparent viscosities under applied strain employed in the present invention, the system may be operated at levels which approach the compressive stress limit, which provide both a fast cutting grinding action and a polishing action on the ground surfaces. The action is attributable to a combination of elastic deformation of the medium and a translation of the work piece surface over the semi-rigid surface of the medium. Elastic deformation is assured by the high levels of applied strain, either compressive or in shear, by the motion imparted. There will be sufficient fluid or plastic flow to provide for conformation of the medium to the surface of the work piece, to provide folding of abrasion debris from the surface into the medium and away from the medium/work piece interface, and to provide the movement of the 30 abrasive within the medium to assure that worn abrasive is removed and fresh abrasive is presented to the working interface. It should be noted that the flow rate is relatively slow and that the elastic relaxation ratio should be fast enough in the medium used to impart a relaxation of about 1 to 10% for each cycle. Thus, for the apparatus of the preferred embodiment is from about 10 seconds to about 1 ten thousands of a second.
The relative motion drive will desirably be operated at relatively small amplitude recurring relative motion, typically an orbital motion, at a relatively high frequency, producing correspondingly high strain rates on the medium. Typical parameters will be an amplitude of motion (e.g., orbital radius) of about 0.010 to about 0.500 inches, preferably about 0.040 to 0.250 inches, and a frequency of from about 5 to 100 Hz, preferably about 12 to 25 Hz.
The system is operated in an open or closed chamber, preferably closed, under applied strain rates such that at least about 50%, particularly about 50 to 99%, and preferably about 80 to 95%, of the deflection of the medium under the conditions of operation occurs by elastic deformation and is elastically recovered, and less than 50%, particularly about 1 to 50%, preferably about 5 to 20%, of the deflection of the medium occurs by fluid or plastic flow.
A preferred medium such as poly(boro-siloxane) carrier matrix is adapted particularly to the requirements of the system is also provided. It employs much higher viscosities and loadings of abrasive than are employed in abrasive flow machining in the prior art, in a poly(boro-siloxane) carrier matrix.
Compared to the fluid or plastic extrusion flow which is the basis of U.S. Pat. No. 5,125,191, the working rates of the elastic deformation of the present invention are both quite rapid and quite fine, permitting the removal of substantial stock and the attainment of a highly polished surface, in many cases in a single operation with a single medium. Where considerable reduction in roughness is required, it is simple to employ a xe2x80x9croughingxe2x80x9d medium followed by a second operation with a xe2x80x9cfinishingxe2x80x9d medium of finer abrasive grit. Rarely will there be occasion to employ more than two media, even to attain the finest surfaces and surface detail and resolution. There is no requirement for sealing the xe2x80x9cchamberxe2x80x9d in which the operation is performed, and a displacer is optional for a great many forms of work piece, and required only for more complex shapes. In addition, because of the elastic behavior of the medium, the conformate requirements of the displacer, if needed at all, are far less demanding than in our prior work.
Other advantages of the present invention will become apparent from a perusal of the following detailed description of presently preferred embodiments taken in connection with the accompanying drawings.